Tissue factor pathway inhibitor (TFPI) is the major inhibitor of the tissue factor/factor Vila (TF/Vlla) catalytic complex that initiates blood coagulation. Therefore, TFPI is a potent anticoagulant protein. The amount of TF within the vasculature system increases in diseases such as atherosclerosis and cancer producing intravascular thrombosis that can result in common and severe diseases such as heart attack, stroke and pulmonary embolism. The long term goal of our proposed studies is to understand the role of TFPI produced by megakaryocytes and expressed on the surface of activated platelets in controlling intravascular TF activity. We hypothesize that platelet TFPI is optimally located and concentrated in order to down-regulate TF activity within a growing blood clot. Thereby, it prevents the formation of blood clots that totally occlude the blood vessel and produce severe, potentially lethal, disease. This hypothesis is based on the following: 1) circulating TF in the blood is a major contributor to clot formation and growth;2) TFPI is the only physiological inhibitor of TF;3) platelets are the major cellular component of clot formation;4) actin and TFPI co-localize within activated platelets;and 5) mice deficient in platelet TFPI have substantial amounts of fibrin deposition in their liver and brain that is not present in mice with platelet TFPI. The experiments proposed here will determine 1) the intracellular location of TFPI within quiescent platelets;2) the biochemical mechanism(s) for transfer of TFPI to the surface of activated platelets;and 3) the physiological function(s) of platelet TFPI in an in vivo mouse model of TFPI deficiency. This proposal is part of a career development plan spanning 5 years. The career development plan includes additional training in the discipline of hematology, mouse pathology and embryogenesis. Continuous interactions with basic scientists at the Blood Research Institute (BRI) of the Blood Center of Wisconsin and the Medical College of Wisconsin will provide 1) access to experienced investigators with expertise in the areas of hematology, coagulation, and pathology, 2) production and evaluation of mouse models, 3) use of in vitro and in vivo systems to study coagulation, and 4) access to internationally and nationally recognized scientists through the visiting lecture series sponsored by the Blood Research Institute. These opportunities will provide additional support and mentoring as Dr. Maroney transitions to an independently funded investigator. RELEVANCE (Seeinstructions): This proposal is relevant to public health as it investigates the role platelet TFPI plays in the down-regulation of the clot forming activities of platelets. Investigation of these mechanisms will enhance our understanding of how platelets can form clots to prevent severe bleeding while controlling clot growth to prevent vessel occlusion that can result in stroke or heart attack or other potentially lethal diseases. (End of Abstract)